Wide-area 20mph zones

By 27th May 2020 5 Comments

Wide-area 20mph zones are proliferating, despite sound evidence presented to the DfT that these zones do not improve road safety, but can: (1) actually increase fatal and serious injury risk for vulnerable road users, (2) do increase vehicle emissions, (3) do waste valuable productive time by lengthening every previously 30mph journey by up to 50%, and (4) do impede the progress of emergency service vehicles.


(1) Casualties:

Research carried out for the DfT (see did not provide statistically significant evidence that wide area 20mph limits reduce vulnerable road user casualties. Joint ABD/ Bridgstock evidence presented to the Scottish Rural, Economic and Connectivity (REC) Committee in February 2019 provided documented evidence of increases in vulnerable road user deaths and serious injuries after the implementation of some 20mph zones, which had replaced previously 30mph ones (

Around 2% of UK urban adult-, and 0.6% of urban child-, pedestrian casualties are fatalities. The Ashton and Mackay curve (below; from DfT Web Publication No.16 – Relationship between Speed and Risk of Fatal Injury: Pedestrians and Car Occupants, TRL, Sep 2010) tells us that for this to the case, the average impact speed in 30mph zones is already below 20mph

The ABD contends that 20mph zones increase driver distraction and workload through increased need to speedometer-watch. This degrades crucial observation, hazard perception, -response and avoidance skills. In tandem, vulnerable road users have been given the highly inappropriate perception that 20mph zones are “safer”; so they behave more incautiously in them. This is a perfect recipe for vulnerable road user accidents in 20mph zones to involve a 20mph impact speed; while at higher speeds, the less speedometer-obsessed, more situationally-aware vehicular road user has time to brake/take evasive action; thereby reducing impact speed or even altogether avoiding an impact.


(2) Emissions:

Per the schematic for NOx emissions shown below, for vehicles to travel at speeds at or below 20mph requires the selection of lower gears, which means higher engine revolutions and increased emissions. Transport for London’s own data (at provides a catalogue (pp.18-32) of curves for various emissions vs average speed by vehicle type scientifically corroborating this fact – which should anyway be blindingly obvious to anyone with even rudimentary automotive knowledge.

(3) Journey Times:

To travel, say, 60 miles at 30mph takes 2 hours. The same journey at 20mph takes 3 hours: 50% longer. The time taken for every previously 30mph journey travelled at 20mph is therefore lengthened by potentially up to 50%. In the year following the imposition of an 80kph blanket speed limit on France’s previously 90kph rural roads, there were no significant casualty reductions (as had been expected by some so-called “road safety” advocates), but the policy did succeed in deflating the French rural economy by 4.4€Bn (see: Similar lost productive time arguments clearly also apply to 20mph zones.


(4) Emergency Vehicle Response Times:

It has already been documented ( that the carpet-bombing of London boroughs with road humps caused more fatalities through increased emergency vehicle response times than the humps had notionally saved. Similar considerations apply to wide-area 20mph zones; many of which are being implemented in areas in which it is impractical for drivers to pull over to let emergency vehicles pass. Likewise, exceeding the speed limit to expedite their progress is no defence against an illegal speed NIP; so highly inadvisable.


  • Brian Gregory says:

    The ABD supports the targetted implementation of 20mph limits where the road engineering configuration is consistent with the use of such a limit; where other measures (e.g., temporary, time-specific limits) cannot be used; and a 20mph limit is appropriate to a very specific local risk environment. We do not condone the indiscriminate, unnecessary and economically costly proliferation of wide-area 20mph zones where the road configuration is not conducive, and the measures are being proposed purely to make motor vehicle use less pleasurable, more difficult, more time-consuming and more costly. Visit to register your support for the ABD’s campaign to prevent the .

  • Brian Gregory says:

    London Mayor, Sadiq Khan’s, has intensified his continuing witch-hunt aimed at the extinction of freedom of private mobility, through the misuse (potentially illegally) of emergency Traffic Orders during the Covid-19 outbreak. These have been used to implement carriageway narrowings, superfluous cycle lane widenings, and so-called “Modal Filters” (roadblocks to everyone else capable of speaking understandable Queen’s English). This has incensed, amongst others, London cabbies: The ABD has in progress a campaign to oppose these fundamentally anti-mobility and anti-democratic policies. Visit:

  • Peter Horton says:

    Fully with you Brian. Have also written to my MP about the folly of the excessive provision for cyclists at the expense of road space for vehicles and asked if they have considered the restraints against commuters cycling – i.e. distance, terrain & weather, and then the need to avoid public transport which boils down just to car use, so should actually be making more provision for car parking and smooth motor traffic flow. From the response I gather that he may actually forward my questions to Grant Shapps, but we can only expect some bland nonsense reply, I fear.

    Best wishes from Ripon.

  • Brian Gregory says:

    James Burdass is encountering a similar mix of ignorance and political apathy in parts of London. One Deputy Labour leader he met knows there are well-founded reservations about their closed-mind, unbounded enthusiasm for 20 limits. But in a 70% Labour controlled Council, they don’t want to be confused with anything as inconvenient as the indisputable facts about their effectiveness. They also seem oblivious that 20 limits near schools are only effective and needed for about 4% of the year when the premises are in use, opening or closing; so time-delineated lower limits are more appropriate. The words horses, water and drink come to mind.

  • Brian Gregory says:

    Sanity has currently prevailed re. the proposals for a city-wide 20mph urban limit in Birmingham. ” We are delighted that the absurd plans for a #20mph limit across #Birmingham have been reigned in by @CharlotteV (pp @grantshapps) ✅ Though Councillor Zaffer will continue to push for them until the voters of #Lozells have the sense to elect someone else”.

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